ext4: rename ext4_has_free_blocks() to ext4_has_free_clusters()

Rename the function so it is more clear what is going on.  Also rename
the various variables so it's clearer what's happening.

Also fix a missing blocks to cluster conversion when reading the
number of reserved blocks for root.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 557c34f..88dc63a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2255,7 +2255,7 @@
 		percpu_counter_read_positive(&sbi->s_freeclusters_counter));
 	dirty_blocks = percpu_counter_read_positive(&sbi->s_dirtyclusters_counter);
 	if (2 * free_blocks < 3 * dirty_blocks ||
-		free_blocks < (dirty_blocks + EXT4_FREEBLOCKS_WATERMARK)) {
+		free_blocks < (dirty_blocks + EXT4_FREECLUSTERS_WATERMARK)) {
 		/*
 		 * free block count is less than 150% of dirty blocks
 		 * or free blocks is less than watermark